The present invention relates to rotary drilling and, more particularly, to a directional drilling technique for providing deviated wellbores at significantly greater inclinations and/or over horizontal distances substantially greater than that currently being achieved by conventional directional drilling practices. The success of such directional drilling should benefit mainly offshore drilling projects as platform costs are a major factor in most offshore production operations. Wellbores with large inclination or horizontal distance offer significant potential for (1) developing offshore reservoirs not otherwise considered to be economical, (2) tapping sections of reservoirs presently considered beyond economical or technological reach, (3) accelerating production by longer intervals in the producing formation due to the high angle holes, (4) requiring fewer platforms to develop large reservoirs, (5) providing an alternative for some subsea completions, and (6) drilling under shipping fairways or to other areas presently unreachable.
A number of problems are presented by high angle directional drilling. In greater particularity, hole inclinations of 60.degree. or greater, combined with long sections of hole or complex wellbore profiles present significant problems which need to be overcome. The force of gravity, coefficients of friction, and mud particle settling are the major physical phenomena of concern.
In the rotary drilling of a highly deviated wellbore into the earth, a drill string comprised of drill collars and drill pipe is used to advance a drill bit attached to the drill string into the earth to form the wellbore. As the inclination of the wellbore increases, the desired weight-on-bit for effective drilling from the drill string lying against the low side of the wellbore decreases as the sine of the inclination angle. The force resisting the movement of the drill string along the inclined wellbore is the product of the apparent coefficient of friction and the sum of the forces pressing the string against the wall. At an apparent coefficient of friction of approximately 0.58 for a common water base mud, drill strings tend to slide into the hole from the force of gravity at inclination angles up to approximately 60.degree.. At higher inclination angles, the drill strings will not lower from the force of gravity alone, and must be mechanically pushed or pulled, or alternatively, the coefficients of friction can be reduced.
In U.S. Pat. No. 4,431,068 to Dellinger and Gravley, there is described a method of drilling a highly deviated wellbore into the earth's crust by a rotary drilling technique wherein a drill string comprised of drill collars and drill pipe is used to advance a drill bit attached to the drill pipe at the lower end of the drill string into the earth and form the wellbore. A first portion of the wellbore is formed to extend essentially vertically into the earth's crust from a surface location thereof to a kick-off point at the lower end of the first portion. A second portion of the wellbore is initiated at the kick-off point which second portion is deviated to a highly deviated attitude from the vertical. The second portion of the wellbore is formed and extended into the earth with the drill string arranged to provide for the drill collars to be located within the first vertical portion and the drill pipe and bit to be located within the second deviated portion of the wellbore. The second portion of the wellbore is extended until the lower end of the lowermost drill collar in the first vertical portion of the wellbore descends to a location of about the kick-off point. Thereafter the drill string is at least partially pulled from the wellbore and additional drill pipe is added thereto below the drill collars after which drilling is continued to extend the second portion of the wellbore further into the earth's crust.